FIG. 1 is a block diagram showing the structure of a prior art optical receiver. In the figure, reference numeral 1 denotes the optical receiver, reference numeral 2 denotes a transmission path, reference numeral 3 denotes a photoelectric conversion means, reference numeral 4 denotes a hard-decision deciding means, reference numeral 5 denotes a demultiplexing means, and reference numeral 6 denotes an FEC (Forward Error Correction) decoding means.
Next, a description will be made as to an operation of the prior art optical receiver.
An optical received signal that reaches the optical receiver 1 by way of the transmission path 2 is converted into an electric received signal by the photoelectric conversion means 3, and is decided as mark or space by the hard-decision deciding means 4. The electric received signal is serial-to-parallel converted by the demultiplexing means 5. Bit errors caused by signal quality degradation in the transmission path 2 and included in the output of the demultiplexing means are corrected by the FEC decoding means 6. An error-corrected parallel received signal is then output from the FEC decoding means.
FIG. 2 is a waveform diagram showing an electric received signal input to the hard-decision deciding means. In a good receiving state with a small amount of degradation in the quality of the optical received signal input to the optical receiver by way of the transmission path 2, the electric received waveform has an eye wide open as shown in FIG. 2(a). On the other hand, in a bad receiving state with a large amount of degradation in the quality of the optical received signal, the electric received waveform has an eye with a small aperture as shown in FIG. 2(b), and the degradation on the mark side becomes remarkable. To avoid this problem, in the hard judgment deciding means 4 a proper decision level according to which the level of the electric received signal is decided as mark or space is set so that a certain bit error rate is provided in a very-bad receiving state (for example, refer to non-patent reference 1).
Non-patent reference 1: ITU-TG.975 recommendation (issued in November, 1996)
Furthermore, following non-patent references 2 to 4 are disclosed as prior art references related to the background of the present invention.
Non-patent reference 2: FEC techniques in submarine transmission systems (OFC2001, paper TuF1, March)
Non-patent reference 3: CONCATENATED FEC EXPERIMENT OVER 5000 KM LONG STRAIGHT LINE WDM TEST BED (OFC99, paper ThQ6, March)
Non-patent reference 4: Third Generation FEC employing Turbo Code For Long Haul DWDM Transmission Systems (OFC2002, paper WP2)
In the prior art optical receiver that is so constructed as mentioned above, the hard-decision deciding means 4 decides (hard-decides) the level of an input electric received signal as either of binary values, i.e., mark or space, and a technique for raising the transmission rate and raising the ratio of redundancy bits included in an error correction code with respect to the information bit is used in order to improve the error correction ability. A problem is however that because there is a trade-off between a rate of increase in the transmission rate and the amount of degradation in the optical transmission quality, and high-speed devices are needed and it is therefore difficult to lower the cost, it is becoming increasingly difficult to improve the transmission quality of high-speed and large-capacity optical receivers.
The present invention is proposed to solve the above-mentioned problems, and it is therefore an object of the present invention to provide an optical receiver that can improve the quality of high-speed and large-capacity transmission without increasing the transmission rate.